based on what I've read and then applied, you don't have to specifically create a database or table, you just initialize it.
Indeed, files are not being written inside /data/db, but they will the first moment you start adding data.
So, I'm taking as an example Twitter, with no db defined, I'm still going to have the db available if I run this code:
$m = new Mongo(); // connect
$db = $m->selectDB("example");
catch ( MongoConnectionException $e )
echo '<p>Couldn\'t connect to mongodb, is the "mongo" process running?</p>';
$updates = file_get_contents( "http://api.twitter.com/". TWITTER_API_VERSION ."/statuses/public_timeline.json" );
$updates = json_decode( $updates );
if ( $updates && is_array( $updates ) && count( $updates ) )
foreach ( $updates as $update )
$db->users->insert( $update );
Hope this was helpful!